An image sensor converts a visual image to digital data that may be represented by a picture. The image sensor comprises an array of pixels, which are unit devices for the conversion of the visual image into digital data. Digital cameras and optical imaging devices employ an image sensor. Image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors.
While CMOS image sensors have been more recently developed compared to the CCDs, CMOS image sensors provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CMOS image sensors have lower manufacturing cost compared with the CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CMOS image sensors. For these reasons, commercial employment of CMOS image sensors has been steadily increasing in recent years.
The CMOS image sensor unit cell comprises a pixel area and a logic area. The pixel area typically contains one of each type of active pixel, e.g., a red pixel having a first photodiode underneath a red optical filter, a green pixel having a second photodiode underneath a green optical filter, and a blue pixel having a third photodiode underneath a blue optical filter. The pixel area may also contain a dark pixel, which is covered with an opaque layer such as an aluminum light shield and employed to measure a background output level in the absence of illumination so that the output of other pixels may be referenced and calibrated. U.S. Pat. No. 6,750,912 to Tennant et al. describes use of a dark pixel and is incorporated herein by reference.
Each photodiode generates charges upon exposure to light. Logic devices connected to the photodiode detect and amplify the charges to generate signals proportional to the incident light. Each pixel comprises at least one photodiode to covert incident photons into electrical charges. Since the area of the photodiode is less than the area of each pixel, a convex-top flat-bottom lens, i.e., an optical lens having a convex surface at the top and a substantially flat surface at the bottom, is typically formed over each photodiode so that photons incident upon a convex-top flat-bottom lens are focused onto the photodiode located underneath. Typically, a color filter is formed between the convex-top flat-bottom lens and the photodiodes to make each pixel color-sensitive, i.e., responsive to photons within a certain wavelength range. The convex-top flat-bottom lens thus acts to focus light over a wide area onto the area of the photodiodes.
The efficiency of each photodiode depends on the quality of the lens system thereabove, that is, the better the lens system collects photons onto the area of the photodiode, the higher the efficiency of the pixel containing the photodiode. Thus, lens systems employing more than one lens has been known in the art. For example, U.S. Patent Application Publication No. 2005/0274968 to Kuo et al. discloses composite lens systems comprising an upper lens and a lower lens. While such a composite lens system can improve the overall sensitivity, or efficiency, of an image sensor pixel, the lower lens structures in Kuo et al. do not provide an ideal lens structure due to the inherent nature of the processing steps employed in the manufacture. Although a flat-top convex-bottom lens with smooth outer surfaces is desired for an ideal lower lens, the lower lens structures provided by Kuo et al. contains at least one step at which the thickness of the lower lens has a discontinuity.
While a flat-top convex-bottom lens is disclosed, for example, in U.S. Patent Application Publication No. 2005/0045927 to Li, integration of such a flat-top convex-bottom lens into a composite lens system containing another lens is challenging since the materials and processes employed in the manufacture of the flat-top convex-bottom lens in Li is not amenable to integration of additional lenses. Of particular concern is the need to provide a material with a higher refractive index that may be patterned into the shape of a lens in a manner compatible with standard CMOS processing steps.
In view of the above, there is a need for an improved composite lens system that enhances the efficiency of an image sensor pixel by providing a higher degree of focusing for incident light, and methods of manufacturing the same.